S. Lamichhane et al. / BIBECHANA 13 (2016) 94-99: RCOST p. 94 (Online Publication: Dec., 2015) BIBECHANA A Multidisciplinary Journal of Science, Technology and Mathematics ISSN 2091-0762 (Print), 2382-5340 (0nline) Journal homepage: http://nepjol.info/index.php/BIBECHANA Publisher: Research Council of Science and Technology, Biratnagar, Nepal Structural and electronic properties of perovskite hydrides ACaH3 (A=Cs and Rb) S. Lamichhane1, B. Aryal1, G. C. Kaphle1,2, 3, N. P. Adhikari1* 1Central Department of Physics, Tribhuvan University, Kathmandu, Nepal 2Tri-Chandra Multiple Campus, Ghantaghar, Kathmandu, Nepal 3Condensed Matter Physics Research Center, Butwal, Rupandehi, Nepal *E-mail: npadhikari@tucdp.edu.np Article history: Received 31 July, 2015; Accepted 20 September, 2015 DOI: http://dx.doi.org/10.3126/bibechana.v13i0.13437 Abstract We have performed structural properties, electronic properties, charge density and potential distribution of CsCaH3 and RbCaH3 using TB-LMTO-ASA approach under local density approximation. Our findings show that both CsCaH3 and RbCaH3 are non-magnetic and then insulators with estimated direct band gaps (M-M) of 3.15 eV and 3.17 eV respectively. Our estimated values of band gap suggest to both materials as better candidate for the high frequency ultra-violet devices. Furthermore, role of corner atoms in perovskite hydrides are also presented in this study. Present calculations agree well with the previous work. ©RCOST: All rights reserved. Keywords: Projected density of states; perovskite; charge density and TB-LMTO-ASA. 1. Introduction Binary alloys of transition and post-transition elements, and also graphene related work have been studied with high effort by many researchers [1-6] but have no band gap. However, various potential applications in optics need significant band gap. The perovskite structure of ternary chlorides is applicable in optics due to their wide band gap [7, 8]. Perovskite compounds and its derivatives are the most popular field owing to their diverse physical properties while going to binary alloy to ternary alloy to perovskite material. Besides this, anomalous dielectric properties of perovskite and phase transition with increasing temperature have made fundamental interest in physics [9-11]. Perovskite hydride compounds belong to Perovskite family, symbolized as ABH3 (A and B represent lighter metals) are well characterized with their optical properties. Perovskite hydrides have become a source of attraction to experimentalists due to presence of lightweight elements, low production cost and S. Lamichhane et al. / BIBECHANA 13 (2016) 94-99: RCOST p. 95 (Online Publication: Dec., 2015) wide range of applications in the industrial area. Alkali and alkaline earth-based complex hydrides are expected to have application for the hydrogen gas storage at certain temperature and pressure [12, 13]. The complexity in structural arrangement of perovskite hydrides and difficulties in establishing proper hydrogen position by x-ray diffraction method in previous attempt [14] motivated us to enter in this burning area of research. As increase in Ca-H distance, narrower the density of states (DOS) on moving from LiCaH3 to CsCaH3, predicted in previous work [15] is insufficient to address the factors for DOS variation and also the compound NaCaH3 has shown absurd behaviour than others in regards to M-H and Ca-H distance. Hence, in the present work efforts have been taken to study the structural, electronic and charge distribution to explore the origin of band gap, contribution of orbitals and role of potentials to bands separation. Moreover, the crystal structure, shape, size, and surface composition of materials plays vital role to control the hydrogen sorption properties for energy storage applications. For the efficient energy carrier, hydrogen should be absorbed and desorbed in materials easily in sufficient quantities in cheaper price. The complex compound based on Alkali- and alkalineearth hydrides are expected to have potential candidates for storing hydrogen at moderate temperatures and pressures [16-17]. This is another motivation of present work to study the electronic behavior of such materials. The paper is organized as follows: in section II, the computational details of the present study are included. Results and discussion are presented in section III whereas section IV provides the conclusions of the present study and finally references are listed at the end of the paper. 2. Computational details The unit cell of the perovskites hydrides CsCaH3 and RbCaH3 crystallize in to the cubic structure with space group Pm-3m. The atomic positions in the elementary cell are at Cs(0.0,0.0,0.0), Rb(0.0,0.0,0.0), Ca (0.5,0.5,0.5) and three hydrogen atoms are at three different positions as H1(0.5,0.5,0.0) , H2(0.5,0.0,0.5) and H3(0.0,0.5,0.5) [8]. With reference to experimental lattice parameter of 4.609 Å for CsCaH3 [18] and 4.547 Å for RbCaH3 [19] unit cell structures thus obtained with the help of xcrysden visualization are shown in figures 1(a) and (b). Fig. 1: (a) Unit cell of CsCaH3 and (b) Unit cell of RbCaH3. S. Lamichhane et al. / BIBECHANA 13 (2016) 94-99: RCOST p. 96 (Online Publication: Dec., 2015) All the calculations are done based on the density functional theory implemented with TB-LMTO-ASA code to carry out comprehensive analysis of structures along with electronic and magnetic properties. The standard local density approximation (LDA) is used to address exchange correlation potential. The concept of Wigner-Seitz spheres for each atom in the unit cell is taken into account to determine the potential of the crystal [20]. We have only included the energetically higher-lying valence states in the self-consistent calculations of the effective crystal potential [21] according to the norms of the TB- LMTO-ASA approach. The core states are treated fully relativistically while the semi-core and valence states as treated semi-relativistically. All the calculations were iterated to self-consistency along with accuracy of 10-6 Rydberg. 3. Results and discussion The new findings from the calculations of band structures, DOS, charge density and potential distributions are described as follows. A. Band Structures The calculated energy bands of perovskites CsCaH3 and RbCaH3 along the high symmetry lines in the Brillouin zone are shown in Fig (2). Fig. 2: Band structures of CsCaH3 (left) and RbCaH3 (right). The horizontal dotted line in the above figures represents the Fermi level. We have found 10 different energy bands in both CsCaH3 and RbCaH3. Zero energy is chosen to coincide with the CBM and VBM both occur at same M point results the direct band gap in both of these perovskite hydrides which indicates that both CsCaH3 and RbCaH3 bear the non-metallic behaviour. Our estimated band gaps of CsCaH3 and RbCaH3 are 3.15 eV and 3.17 eV which indicates that CsCaH3 and RbCaH3 are insulators. Our results of band gap well agree with previous generalized-gradient approximation based theoretical calculations [8]. Resulting band gap of these perovskites meet the requirement for the applications in high frequency ultra-violet device [22]. S. Lamichhane et al. / BIBECHANA 13 (2016) 94-99: RCOST p. 97 (Online Publication: Dec., 2015) B. Density of States To observe the density of states, we have taken same value of the lattice parameter as in band structures calculations. The total DOS and PDOS of CsCaH3 and RbCaH3 are shown below. The total density of states are categorized into four different parts and the contributions of different states can easily be seen from the PDOS analysis. Fig. 3: Total DOS and PDOS of CsCaH3(left) and RbCaH3(right). The Fermi energy line is in the valence band which supports the non-metallic behaviour. In both CsCaH3 and RbCaH3 Ca-p and H-s states are located in the vicinity of the Fermi level at the valence band. In CsCaH3 Ca-s and −p states are well-separated where as in RbCaH3 Ca-s and −p states are not well- separated. The symmetric nature of spin up and spin down DOS indicate the non-magnetic behavior of CsCaH3 and RbCaH3. The contribution to the valance band is maximum by s states of hydrogen and less contributions is seen from Ca and Cs. The contribution of Ca and H atoms to the DOS is major in the valence bands while Cs/s, Cs/p are responsible for that in the conduction band. Further, one valance electron of Cs (or Rb) in CsCaH3 ( or RbCaH3) fill the partially occupied s-orbital of one hydrogen and remaining two partially filled orbitals of hydrogen is filled by two valence electrons of calcium. Since the valence electrons are completely filled, CsCaH3 and RbCaH3 both acts as an insulating materials. C. Charge density and potential distribution The charge density distribution and chemical bonding behaviour of MCaH3 is important to check the stability of these materials for hydrogen storage. The charge density plot of CsCaH3 and RbCaH3 obtained by using the same lattice parameters as in DOS and band structures calculations are shown in figure (4). S. Lamichhane et al. / BIBECHANA 13 (2016) 94-99: RCOST p. 98 (Online Publication: Dec., 2015) Fig. 4: The charge density distribution of CsCaH3 and RbCaH3. From figure 4, we have found that there are five peaks along the corners and faces which shows the maximum distribution of the charges at the corners and faces. About 10-40 % of total charge occur along the corners and faces which has been shown by a high peaks lines while the distribution of the charges is negligible at other interstitial sites. This is the site at which adsorption and desorption of hydrogen takes place, indicating that such materials can be used for hydrogen storage purposes. The potential distribution plot of CsCaH3 and RbCaH3 are shown in figure (5) . Fig. 5: Potential distribution in CsCaH3 (left) and RbCaH3 (right). From figure 5, we can find that the potential is maximum in the vicinity of core whereas at the interstitial region the potential is found to be constant (flat). Our results agree well with the muffin-tin potential in which the potential is maximum near the core electrons and almost flat in the interstitial space. This kind S. Lamichhane et al. / BIBECHANA 13 (2016) 94-99: RCOST p. 99 (Online Publication: Dec., 2015) of potential distribution fully supports the maximum distribution of charge near the corner of crystal structure. 4. Conclusions In this present work, we studied the band structures, density of states, charge density and potential distribution of perovskite hydrides CsCaH3 and RbCaH3 by using TB-LMTO-ASA. Taking the experimental lattice parameters, we calculated the electronic and magnetic properties of CsCaH3 and RbCaH3. The calculated DOS shows that CsCaH3 and RbCaH3 are non-magnetic. The direct band gap of 3.15 eV found in CsCaH3 and 3.17 eV in RbCaH3 meet the requirement for the application in high frequency ultra-violet device. The charge density plot of CsCaH3 and RbCaH3 indicate that the corner atom contributes mainly to increase charge density which also support from the potential distribution curve. From potential distribution of CsCaH3 and RbCaH3, we found that potential is maximum at core whereas flat at interstitial region. This may be the vital site to store the hydrogen and hence system acts as hydrogen storage materials. Finally we can conclude that TB-LMTO-ASA method can be used as a convenient method for studying the band structures and density of states of different solids. Our results may be useful to study the other various types of elemental solids and perovskites in future. Acknowledgments We are grateful to ICTP/OEA NET-56 program of Trieste, Italy for the partial support. References [1] B. I. Min, T. Oguchi, H. J. F. Jawsen, A. J. Freeman, Phy. Rev. B, 12 (2003) 1091. [2] M. Fatmi, Rom. Journ. Phys. 56, (2011) 937. [3] S. Lamichhane, N. Pantha, N. P. Adhikari, BIBECHANA, 11 (2014) 113. [4] K. T. Chan, J. B. Neaton, M. L. Cohen, Phys. Rev. B, 77 (2008) 235430. [5] S. Pandey, G. C. Kaphle, N. P. Adhikari , BIBECHANA, 11 ( 2014) 60. [6] G. C. Kaphle, S. Ganguly, R. Banerjee, R. Khanal, C. M. Adhikari, N. P. Adhikari, A. Mookerjee Condens. Matter, 29 (2012) 295501. [7] R. Hua, B. Lei, D. Xie, C. Shi, J solid State Chem., 175 (2003) 284. [8] P. Vajeeston, P. Ravindran, H. Fjellvag, The J. Chem. 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